Carbon nanotubes have now been a subject of intense research for approaching two decades. Although a short time relative to most conventional materials, much hype about the intrinsic properties of this material has now been substantiated by experiment. The results are conclusive that carbon nanotubes are truly phenomenal materials with highly desirable mechanical, electrical and thermal properties. Furthermore, multiwall carbon nanotubes (MWNTs) have emerged as the most economically viable and abundant form of carbon nanotubes, and therefore the most likely candidate for application. The key materials engineering challenge remains in effectively transferring their properties to macro-scale materials in the form of composites. It is here that research merges with application. This dissertation has therefore been directed to focus on carbon nanotube composites in an applied sense. Here, the state of the art is reviewed, and experimental results of carefully selected composite systems, studied in detail for (1) mechanical, (2) electrical and (3) thermal properties, are presented and discussed. In terms of mechanical properties, the effects of MWNTs for augmentation of the tensile properties of PAN-based carbon fiber, and fatigue performance of poly(methyl methacrylate) are investigated and reported. In MWNT composite PAN-based carbon fiber, the formation of an ordered interphase layer sheathing the nanotubes was observed in fracture surfaces, which indicated a clear importance of their function to template the growth of carbon formation in the PAN-based matrix fiber. These structures open up a route to nano-scale tailorability of the crystallographic morphology of the composite fibers. Large improvements in fatigue performance were observed in MWNT/PMMA composites compared to MWNT/chopped carbon fiber composites, and attributed to the nanometer scale dimensions of the MWNTs enabling them to mitigate submicron damage such as polymer crazing. In terms of electrical and thermal properties, MWNT/epoxy composites were superior to MWNT/carbon black composites. Furthermore, extremely large improvements in the thermal conductivity of epoxy were observed for epoxy-infiltrated aligned MWNT arrays. The alignment of the MWNTs was shown to play a dominant role in enabling the improvement. Finally, these results, in concert with the literature are discussed in terms of the application of carbon nanotubes in engineering materials.

We develop homogenization schemes for the effective mechanical and acoustical properties of linear viscoelastic inclusion reinforced composites. Generic procedures are studied and proposed, starting from the statics and elastic materials, followed by viscoelastic behaviors and the quasi-static approximation, to end up with dynamic viscoelastic models. For elastic composites, our main contributions regard homogenization schemes for coated inclusion reinforced materials. An original two-level recursive procedure is proposed. At each level, a homogenization model suitable for two-phase composites with aligned reinforcements is required. That makes the procedure very generic. The latter is used as stand-alone (i.e. for three-phase composites) or to achieve the first step of a two-step approach for multiphase materials (e.g. composites with misaligned coated fibers). With the quasi-static approximation, i.e. when inertial effects are ignored, we generalize the generic two-step and two-level procedures to predict both time dependent and harmonic effective mechanical properties (storage and loss moduli) of linear viscoelastic multiphase composites. The static and quasi-static models however cannot account for attenuation of acoustic waves due to scattering by the inclusions. Effective acoustic properties (phase velocities, attenuation factors) of viscoelastic inclusion reinforced composites are addressed by dynamic schemes. We propose a generic framework for the development of such homogenization models. It generalizes and unifies within a single formulation several methods of the literature. An independent-scattering model is derived. The link between dynamic schemes in the long-wave region and static models is shown. Finally, these homogenization schemes are integrated in a finite element model based on a discontinuous Galerkin formulation of the Helmholtz equation to simulate acoustic problems involving viscoelastic composite materials.

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Polymer electrolyte membrane fuel cells (PEMFCs) exhibit encouraging potential as an enabling technology for the Hydrogen Economy. Currently an important barrier to commercialization is the cost associated with existing PEMFC materials; this project???s goal was to investigate alternative materials for PEMFC bipolar plates. Conductive thermoplastic materials offer the promise of low density, low cost processing, and inexpensive resins, and were the focus of material development for PEMFC bipolar plate applications. In order to develop a thermoplastic bipolar plate this study utilized the combination of a low cost injection moldable commodity polymer resin, and low cost carbon materials as conductive fillers. The materials selected and tested included; a polypropylene copolymer; acetylene carbon black; Vulcan carbon black; and short carbon fiber. The components were combined in a twin screw extruder and injection molded into samples for testing. The result was a spectrum of composite samples with a range of filler loadings from 0 to 60 wt% and varying filler type ratios. Synergy between the different carbon types was achieved which led to better physical properties, specifically conductivity. The novel blends produced were tested for electrical conductivity, mechanical properties, rheology, microscopy, and actual plates were made and tested in a single cell PEMFC. These trials enabled discussion around the feasibility of the materials with respect to processability, cost, and performance (both in the fuel cell and in potential applications). The most significant results were measured using a composite blend with 54 wt% filler loading and a 1:1:1 filler ratio. Mechanical results achieved 68% and 100% of the industry targets for tensile and flexural strength, respectively. Tensile strength attained 27.7 MPa and flexural strength measured 82.8 MPa. Electrical conductivity results for the same samples varied between the two methods of measurement used. Using a fuel cell industry recommended procedure 2.2 S/cm was achieved and using a four point ASTM measurement technique 12.0 S/cm was reported. These values represent 3% to 12% of the industry target. Actual 16 cm2 fuel cell plates were produced, fuel cell hardware constructed and assembled, and the power output was found to be 51% relative to graphite plates. Thermoplastic bipolar plates for PEMFCs made of composite materials is promising, but optimum filler loading that balances all properties is still required in order to achieve conductivity targets. Nevertheless this study has demonstrated that conductive thermoplastic bipolar plates can be produced via injection molding.